1. Field of the Invention
The present invention relates to a process for the tubular blown film extrusion of a thermoplastic resin and more particularly and in a preferred embodiment, to an improvement in a process for the tubular blown film extrusion of a low-strain hardening polymer, such as a low pressure-low or high density ethylene copolymer.
2. Description of the Prior Art
In a conventional technique for forming tubular blown film suitabkle for the fabrication of bags and the like, a film-forming polymer, such as polyethylene, is extruded through an annular die arranged in an extrusion head so as to form a tube of molten polymer having a smaller outer diameter than the intended diameter of the eventually-produced film tube. The molten tube is drawn radially in its path upward from the die lips of the annular die by a force created by the differential pressure resulting from the cooling air flow from a venturi type lip air ring and the internal bubble pressure. The film tube is typically drawn radially only about one half to one inch prior to being contacted by the cooling air flow, and prior to contact it is usually drawn down to no more than half of its thickness at the die exit. The subsequent crystallization kinetics and rheological dynamics influence the resultant film optical and physical properties. Illustrative of prior art techniques utilizing the venturi type cooling modes and the effects upon film properties can be found for example in U.S. Pat. Nos. 3,167,814, 3,210,803 and 3,548,042. After cooling to solidify the molten tube, the tube is directed through flattening means such as a collapsing frame and a pair of driven rollers, to flatten the extruded film tube. Between the point of extrusion and the terminus of the flattening means, the film tube is expanded by means of air or some other gaseous medium to thereby form a film bubble and the film bubble is maintained by the gas trapped within the expanded film bubble between the die and collapsing means. The driven nip rolls draw the molten tubular film away from the annular die at a speed greater than the extrusion speed. This, together with the radial expansion of the molten film bubble, decreases the film thickness and orients the blown film in both the machine and transverse directions. The degree of radial expansion and the speed of the driven nip rolls may be controlled to provide the desired film thickness and orientation. The location at which the molten film bubble essentially completely solidifies is referred to in the art as the "Frost Line".
Thermoplastic materials which may be formed into film by the tubular blown film process include polymers of olefins such as ethylene, propylene, and the like. Of these polymers, low density polyethylene (i.e., ethylene polymers having a density of about 0.94 g/cc and lower) constitutes the majority of film formed by the tubular blown film process. As used herein, the term ethylene polymers includes ethylene homopolymers, and copolymers of ethylene with one or more comonomers. Conventionally, low density ethylene polymers have in the past been made commercially by the high pressure (i.e., at pressures of 15,000 psi and higher) homopolymerization of ethylene in stirred and elongated tubular reactors in the absence of solvents using free radical initiators. Recently, low pressure processes for preparing low density ethylene polymers have been developed which have significant advantages as compared to the conventional high pressure process. One such low pressure process is disclosed in commonly-assigned, copending U.S. application Ser. No. 012,720 filed Feb. 16, 1979 (a foreign-filed application corresponding thereto has been published as European Patent Publication No. 4647) and U.S. Pat. No. 4,302,565. It has also been recently determined that resins similar to the above low pressure process resins have been made in modified conventional LDPE equipment; e.g. tubular or stirred reactor equipment. Such resins have similar extensional viscosity indexes and the process of this invention will also apply to those resins.
The above-identified U.S. Patent and application disclose a low pressure, gas phase process for producing low density ethylene copolymers having a wide density range of about 0.91 to about 0.94 g/cc and a melt flow ratio of from about 22 to about 36 and which have a relatively low residual catalyst content and a relatively high bulk density. The process comprises copolymerizing ethylene with one or more C.sub.3 and C.sub.8 alpha-olefin hydrocarbons in the presence of a high activity magnesium-titanium complex catalyst prepared under specific activation conditions with an organo aluminum compound and impregnated in a porous inert carrier material. The copolymers (as applied to these polymers, the term "copolymers" as used herein is meant to include polymers of ethylene with 1 or more comonomers) thus prepared are copolymers of predominantly (at least about 90 mole percent) ethylene and a minor portion (not more than 10 mole percent of one or more C.sub.3 to C.sub.8 alpha-olefin hydrocarbons which should not contain any branching on any of their carbon atoms which is closer than the fourth carbon atoms. Examples of such alpha-olefin hydrocarbons are propylene, butene-1, hexene-1, 4-methyl pentene-1 and octene-1.
The catalyst may be prepared by first preparing a precursor composition from a titanium compound (e.g., TiCl.sub.4), a magnesium compound (e.g., MgCl.sub.2) and an electron donor compound (e.g., tetrahydrofuran) by, for example, dissolving the titanium and magnesium compounds in the electron donor compound and isolating the precursor by crystallization. A porous inert carrier (such as silica) is then impregnated with the precursor such as by dissolving the precursor in the electron donor compound, admixing the support with the dissolved precursor followed by drying to remove the solvent. The resulting impregnated support may be activated by treatment with an activator compound (e.g. triethyl aluminum).
The polymerization process may be conducted by contacting the monomers, in the gas phase, such as in a fluidized bed, with the activated catalyst at a temperature of about 30.degree. to 105.degree. C. and a low pressure of up to about 1000 psi (e.g., from about 150 to 350 psi).
The tubular blown film extrusion process may be employed to form a film from low pressure-low density ethylene polymers. For example, a process for forming film from one such low pressure-low density ethylene copolymer is disclosed in commonly-assigned, U.S. Pat. Nos. 4,243,619 and 4,294,746. However, it has been found that in some cases the film production rates obtained in tubular film processes with low strain hardening polymers and particularly with low pressure-low density ethylene copolymers, utilizing conventional cooling devices and techniques which cool with air rings of the type which direct air flow in a manner such as to create a reduced pressure zone e.g., by a venturi effect, are low compared to the rates achievable in commercial tubular blown film processes using conventional high pressure-low density polyethylene. Furthermore, the crystallization kinetics and rheological dynamics behave in such a manner that the haze and gloss of said films as measured by ASTM D1003 and ASTM D2457 are typically in the low clarity, low gloss regime.
As is known, the rheology of conventional high pressure-low density polyethylene resins (HP-LDPE) is decidedly different than the rheology of low pressure-low density ethylene copolymers (LLDPE). For a description of these differences refer to U.S. Pat. No. 4,243,619 issued Jan. 6, 1981.
An important difference relates to the strain hardening extensional rheology of LLDPE polymers. In general, LLDPE polymes have low strain hardening extensional behavior whereas the materials produced by high-pressure polymerization have relatively high-strain hardening extensional behavior.
Specifically, the properties of low pressure-low density ethylene polymers are such that commercially desirable high film production rates have not been achieved without film bubble instability. Stated conversely, film bubble instability problems prevent the commercially desirable high film production rates from being obtained in blown film extrusion processes including those utilizing venturi action for cooling and film tube expansion with low pressure-low density ethylene polymers. Among the reasons for such failures, it it believed, is the extensional rheology of low pressure-low density ethylene polymers. In comparison to conventional high pressure-low density polyethylene, certain low pressure-low density ethylene polymers are softer and exhibit less melt strength in extension. As a result, when these low pressure-low density ethylene polymers are extruded from the die in tubular blown film processes and are externally cooled by blowing air against the resin with venturi type action as mentioned previously, the film bubble becomes unstable by the increased cooling required by increased throughput rates. In other words, film bubble instability results at higher throughput rates since such rates require more heat transfer in the cooling process which is usually accomplished by increasing the amount and/or velocity of cooling air which in turn causes bubble instability e.g. the film bubble becomes non-uniform due to the extensional behavior of these low pressure-low density ethylene polymers.
Thus one of the major rate limiting factors in the extrusion of LLDPE blown film is reduced bubble stability due to the inherent low-strain hardening extensional behavior of the polymers. This is compounded by the large die gaps which are normally used to eliminate melt-fracture (see U.S. Pat. No. 4,243,619). Melt-fracture is eliminated by the use of large die gaps, but unfortunately, bubble cooling rates may also be reduced, resulting in higher frost line heights and even greater bubble instability. The net result is usually reduced extrusion rates from those attainable with HP-LDPEs.
This rheological behavior, in conjunction with the wide die gaps now commonly employed, is also believed to be partly responsible for the typically high haze and low gloss values of films made with LLDPEs using conventional bubble cooling techniques. Under typical conditions, low stress levels exist in the melt due partially to the low-strain hardening behavior of the polymer. This reduces the tendency to "draw out" surface imperfections in the film and also tends to reduce the extent of stress induced crystallization both of which are factors which increase haze and reduce gloss. In addition, the proclivity of LLDPEs to melt fracture tends to produce an initially rougher film surface than is found in conventional HP-LDPEs. In other words, higher initial surface roughness ensues as a result of the shear rheology and is in turn removed to a lesser degree due to the extensional behavior. Furthermore, fewer stress induced polymer crystallites form due to the lower levels of stress stemming from the non-strain hardening extensional rheology. As a result, high clarity and industrial clarity films have not heretofore been satisfactory fabricated from LLDPEs using conventional tubular film making technology. This limits the utility of LLDPE resins in clarity film markets.